Aboslute clustering effects on electron attachment

Electron attachment plays an important role in radiation chemistry, for example in DNA damage and ozone depletion. Detailed understanding and quantification of electron attachment processes in isolated molecules and condensed environments is therefore essential to model radiation effects on the nanoscale. My EPSRC CAF probes electron attachment dynamics and reactive pathways in selected biomolecular clusters, building on recent advances such as the observation of electron driven proton transfer in Watson Crick pairs [Bowen et al. ChemPhysChem 11 (2010) 880]. However, relatively little is known about how clustering modifies the absolute probabilities for electron attachment induced processes. While theoretical calculations by my collaborators Fabrikant and Gorfinkiel [J. Chem. Phys. 136 (2012) 184301] have provided evidence for strong enhancements in specific cluster configurations, absolute experimental data for electron attachment to clusters are extremely rare. This project is centered on developing an original technique to produce neutral mass-selected beams with known target density for electron attachment experiments. The method involves neutralization of mass-selected cluster anions by electron photo-detachment from specific weakly-bound anionic states, with minimal change in stability and hence dissociation. The project will provide a breakthrough in quantifying the effects of the local chemical environment on electron attachment induced processes.

This project will provide key input data for simulating radiation-induced (bio-) molecular damage and nano-scale dosimetry in aqueous environments with applications in radiotherapy. Despite the fact that dissociative electron attachment has been known to initiate DNA lesions for over 10 years [Sanche and co-workers, Science 287 (2000) 1658], low-energy electron interactions are not included in current radiotherapy treatment planning. Dramatic advances in radiation delivery systems (for example using ions beams and new methods in intensity modulated x-ray radiotherapy) have highlighted the importance of more accurate simulations, while computing power no longer prohibits the practical applications of Monte Carlo methods. Advancing the efficacy of cancer therapy methods is essential in view of the aging populations of many countries. Therefore, the development of optimized radiation track simulations applying the most reliable electron attachment (EA) and dissociative electron attachment (DEA) cross sections available will have real benefits for society. The absolute cross sections for electron interactions with water clusters measured during this project will be integrated into Garcia's simulations (LEPTS, currently based on gas-phase cross sections) as a part of a new strategy to account for the hydrogen-bonded environment. Garcia has strong connections with oncologists who are currently carrying out trials comparing clinical radiotherapy treatment plans with damage distributions derived using LEPTS.

Furthermore, the project will contribute to understanding environmental chemistries. The small neutral and charged water clusters studied in this project are prevalent in the atmosphere. In addition, they can provide model systems to help us understand processes in larger clusters, ice crystals, and droplets. Evidently the diverse phases of water play key roles in atmospheric dynamical processes and simulations of their interactions with low energy electrons (the most abundant secondary products of ionizing radiation) require absolute cross sections. Currently available experimental cross sections for electron attachment to selected water clusters are limited to non-dissociative electron attachment [Dubov and co-workers, JETP letters 86 (2007) 520]. To our knowledge, absolute experimental data is not available for DEA to doped water clusters. Our choices of doped cluster targets (hydrated CF3Cl) are known to play roles in ozone depletion chemistries in polar stratospheric clouds [Tachikawa and co-workers, PCCP 10 (2008) 2200]. While the specific complexes studied in this project may not have a significant presence in these environments, they can impact on the residence time of CF3Cl at lower altitudes and hence in the molecule's transport to polar stratospheric clouds. Enhanced DEA cross sections for CF3Cl in hydrated clusters can therefore have significant implications for modeling and assessing ozone depletion.

The project will also have impact in plasma technology and mass spectrometry. Firstly, the absolute data will have applications in simulating technological atmospheric pressure plasmas. These have developing applications in surgery and water clusters provide key sites for chemical reactivity in these plasmas. Secondly, the original experimental method to produce controlled beams of mass-selected neutral clusters can have numerous applications beyond the scope of this project. As well as diverse variations of the proposed crossed-beam experiments (e.g. using ultrafast lasers), beams of selected neutral clusters can be applied to initiate controlled chemistry on surfaces and hence develop nanotechnologies. The industrial partnership with Hiden Analytical recognises the potential of this proof-of-concept to stimulate a major new line of experimentation on quadrupole-controlled neutral beams with future commercial opportunities in specialised instrumentation.